Hi all, I have 2 FW tanks one main tank at about 190 gal. and another at 30 gal. When I unscrew the cap on the main tank to fill, I notice some vacume on the cap. I checked the vents and all seem clear. Any thoughts on where to start. My boat only runs off the tanks-no city water hook-up. Pump seems to be working fine with either tank. Any thoughts would be appreciated. Thanks